DCs are the centre-piece of the immune system. They orchestrate the type and intensity of the immune mechanisms that prevent disease. Given the heterogeneity of these immune mechanisms, it is not surprising that the DC subsets that control them are equally diverse. Prom the first description of DCs as antigen presenting cells [1], there has been a concerted effort to categorise the phenotypically distinct subsets and ascribe functional differences. Such studies unveiled that DCs could be divided into subsets by their expression of CD8 into CD8+, CD81nt (intermediated expression) and CD8− populations [2-4]. The CD8− populations can be further divided into CD4−CD8− (double negative DN) and CD4+CD8− populations [5, 6]. Segregation of DCs by these markers has uncovered significant differences in anatomical localization [7-9], antigen uptake and processing [10, 11] as well as cytokine production [12-15]. Although maturation/activation states of the subset, as well as the nature and amount of the antigen encountered can play a large role in dictating the functional outcome, some functional ascriptions have been made. For example, it is believed that CD8+ DCs (found mainly in the T-cell areas) are preferentially involved in cross-priming CTL responses [16-18], whereas CD8− DCs (mainly found in non T-cell areas) may be more associated with potentiating T helper type 2 [17] and B-cell responses [7, 9].
Along with phenotypic distinctions associated with the expression of CD8, the origin of the DCs can have significant impact on the induction of cytokine as well as T and B-cell responses. This is particularly evident when comparing DCs isolated from mucosal versus peripheral tissues. For example, DCs isolated from Peyer's Patches stimulate the production of more IL-4 and IL-10 but less IFN-γ [19]. In fact these IL-4 and IL-10 inducing cells are far more potent in stimulating allogenic T-cell proliferation compared with splenic DCs [19]. CD11b+ CD8− DCs isolated from the Peyer's Patches also preferentially secrete IL-6 and induce the secretion of IgA from naïve B-cells [20]. DCs from the mesenteric lymph nodes preferentially enhance T-cell expression of the mucosal homing receptor LPAM-1 as well as chemokine receptor CCR9 [21, 22].
Targeting DCs via non-lineage specific cell surface markers such as, MHC class II [23], CD11c [24], CD80/CD86 [25], toll-like receptors [26], DEC205 [27] and immunoglobulin Fc-receptors [28, 29], can enhance the systemic response to antigen. These strategies however, fail to deliver antigen at the levels required for the induction of mucosal immunity. One approach to mucosal vaccination attempted to target M-cells (in gut lining) that shuttle antigen from the mucosal surface to underlying lymphoid tissue [30, 31]. Although somewhat promising, these techniques have so far failed to overcome the problems faced in effectively delivering antigens for the efficient induction of mucosal immune responses.
To combat the constant threat of infection, the mucosa is littered with a diverse assortment of specialised lymphocyte populations. Positioning of these lymphocytes throughout the mucosa is critical for proper effector function. Thus, lymphocyte positioning is tightly regulated by the coordination of a number of unique homing receptors. These include the specific expression of cellular adhesion molecules VCAM-1, ICAM-1, MAdCAM-1 and E-cadherin. Lymphocytes use heterodimeric complexes of the integrin family such as α4β7 (LPAM-1) [32-34], α4β1 (LPAM-2) [35], and αEβ7 [36, 37] to bind to these adhesins and move through the tissue in search of antigen presenting DCs. Once lymphocytes encounter DCs, they form close interactions that are required for antigen recognition. Indeed, these interactions are so close that the DC can acquire membrane components (including bound antigens) from the lymphocyte [38-40]. It is therefore possible that lymphocyte populations programmed with unique and well-characterised homing potentials could be used as carriers to deliver antigen to particular DC populations. Such a mechanism may overcome the lack of suitable markers for the efficient delivery of vaccine antigens to mucosal DCs and may overcome the normal barriers to the induction mucosal IgA responses.
Given that DCs are constantly bombarded with lymphocytes that do express unique homing receptors, the present inventors have proposed a 2-step targeting model whereby lymphocytes could “relay” antigen to sites of immune induction. The mucosal homing receptor lymphocyte Peyer's Patch adhesion molecule-1 (LPAM) was investigated. LPAM is upregulated on mucosally targeted lymphocytes where it facilitates the interaction with the mucosal address in cellular adhesion molecule-1 (MAdCAM) [32, 41]. The present inventors provide a model for lymphocyte mediated delivery of antigen to mucosal lymphoid tissues of the gut as well as to peripheral lymphoid tissues.